Three men have been arrested in connection with the suicide bombings in London on 7 July 2005.
The 7 July bombers attacked a London bus and three Tube trains
Two men, aged 23 and 30, were arrested shortly before 1300 GMT at Manchester Airport when they were due to catch a flight to Pakistan.
A third man, aged 26, was arrested at a house in Leeds shortly after 1600 GMT.
The men were held on suspicion of the commission, preparation, or instigation of acts of terrorism. Fifty-two people were killed by four bombers on 7 July.
Mohammad Sidique Khan, 30, Shehzad Tanweer, 22, and Germaine Lindsay, 19, detonated bombs on three Tube trains and Hasib Hussain, 18, attacked a bus.
Thursday's arrests are the first major ones since the attacks.
Two of the arrested men are at Paddington Green police station, central London, and the third is believed to be on his way there.
They will be interviewed by officers from the Metropolitan Police's Counter Terrorism Command.
Police are searching five homes in the Beeston area of Leeds
Searches are being carried out at five houses in the Beeston area of Leeds, West Yorkshire Police said.
The addresses are in Cardinal Road, Colwyn Road, Firth Mount, Tempest Road, and Rowland Place.
Home Secretary John Reid said the arrests were part of an "ongoing operation" into the events of 7 July.
He said: "The police will put all the appropriate information into the public domain."
Mr Reid added that it was important to avoid speculation.
Tanweer and Hussain had both been living in Beeston when the attacks were carried out and Khan grew up in Beeston.
Tanweer lived in Colwyn Road with his parents.
A flat and a separate business premises are also being searched in east London. It is understood that the business is in Whitechapel and the flat in Bromley-by-Bow.
Scotland Yard said the arrests were part of a pre-planned, intelligence-led operation and also involved the West Yorkshire Police Counter Terrorism Unit.
Ch Supt Mark Milsom, of West Yorkshire Police, said it had not been a high profile operation and unarmed officers were carrying out the searches.
'No danger to public'
He said the searches may take "some time" but they were not expecting to find firearms or bomb-making equipment.
"There's no danger, as we know it, to the public at this time," he said.
Lord Carlile QC, the independent reviewer of terrorism legislation, said: "Anybody who imagined that this had simply been treated as four lone wolves, or a pack of wolves on 7 July 2005 was very wrong."
He said a "rigorous hunt" was going on for everyone connected with the attacks and nobody involved could "lie easy in their beds".
A Scotland Yard spokesman said: "Detectives have continued to pursue many lines of inquiry both here in the UK and overseas.
"This remains a painstaking investigation with a substantial amount of information being analysed and investigated.
"As we have said previously, we are determined to follow the evidence wherever it takes us to identify any other person who may have been involved, in any way, in the terrorist attacks.
"We need to know who else, apart from the bombers, knew what they were planning. Did anyone encourage them? Did anyone help them with money, or accommodation?"